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Relief of the internal stress in ternary boron carbonitride thin films
Applied Surface Science 191 (2002) 338±343
Relief of the internal stress in ternary boron carbonitride thin ®lms Deyan Hea,*, Wenjuan Chenga, Juan Qina, Jinshun Yuea, Erqing Xiea, Guanghua Chenb b
a Department of Physics, Lanzhou University, Lanzhou 730000, China College of Materials Science and Engineering, Beijing Polytechnic University, Beijing 100022, China
Received 30 October 2001; accepted 27 March 2002
Abstract Ternary boron carbonitride (BCN) thin ®lms were deposited by radio frequency reactive sputtering. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) absorption measurements suggested that the ®lms are atomic-level hybrids composed of B, C and N atoms. The structure of the samples is graphitic BC2N. High compressive stresses were observed in the ®lms deposited on silicon and fused silica substrates. When the ®lms were peeled off the substrates, the absorption bands in the FT-IR spectra shift to lower wavenumbers due to the stress relief. The compressive stresses caused buckling of the ®lms, and the buckling patterns were related to the used substrates. For the samples deposited on Si wafer, a branching type buckling pattern was observed, while a telephone-cord morphology was noted for the samples deposited on fused silica plate. The telephone-cord morphology can be explained based on the buckling-driven delamination theory proposed by Oritiz and Gioia [J. Mech. Phys. Solids 42 (1994) 531]. # 2002 Elsevier Science B.V. All rights reserved. PACS: 68.35.Gy; 68.55.Jk Keywords: Ternary boron carbonitride thin ®lms; Compressive stress; Buckling pattern; Buckling-driven delamination theory
1. Introduction Although graphite and hexagonal boron nitride (hBN) are both hexagonal close-packed in structure and have similar atomic size and crystal constant, they differ greatly in properties: graphite is a good conductor, while h-BN is an insulator. The structural similarity between the two materials inspires materials scientists to develop a ternary BCN compound. The structure of the compound is expected to be graphite-like, and *
Corresponding author. Tel.: 86-931-891-2546; fax: 86-931-891-3554. E-mail address: [email protected] (D. He).
its properties, which may be between those of graphite and h-BN, like a semiconductor with adjustable band gap by controlling the atomic composition and arrangement [1,2]. On the other hand, since ternary BCN compounds can be prepared in a wide composition range, the materials are also expected to be used as protective coatings in tribology and optics [3] and precursors for the preparation of heterodiamond under high-pressure high-temperature conditions [4]. Moreover, it was shown that the intercalation property of BCN compounds is between the graphite and h-BN, and the materials could be applied for rechargeable lithium batteries [5].
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 2 5 4 - 4
D. He et al. / Applied Surface Science 191 (2002) 338±343
Much effort has been made to prepare BCN compounds. It was reported that the material could be synthesized by various techniques [6±9] and the resultant samples were stoichiometrically varied: BCN, BC2N, BC3N, and B2C5N, etc. Some published works also focused on the structural characterization of the materials. With regard to the applications of BCN compounds as protective coatings in tribology and optics, their mechanical behavior and stability are of primary importance. High compressive stress has been found in BCN ®lms [10]. Since the compressive stress may affect the adhesion properties of the materials on substrates and thereby their applications as protective coatings, it is necessary to have an understanding of the stress relief within the materials. In this paper, BCN thin ®lms were prepared by radio frequency reactive sputtering method. The microstructure and the residual stress relief within the samples were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) absorption, and scanning electronic microscope (SEM). 2. Experimental The BCN ®lms used in this work were deposited by reactive sputtering technique [11]. The target was a sintered h-BN disk with a diameter of 10 cm. The sputtering gas was a mixture of methane and argon. The fused silica plate and high resistance Si wafer were used as substrates. Before the deposition, the substrates were rinsed ultrasonically in acetone, ethanol and de-ionized water and then dried by N2 blowing. For Si wafer, a dip in a solution of 5% HF was done just before mounting it onto the acceptor in order to remove the surface thin oxide layer. The ¯ow rate ratio of CH4/Ar was 15% with the total ¯ow being 20 sccm. The working pressure was kept at 1 10 2 Torr. The density of the radio frequency power was 3.84 W/cm2 and the substrate temperature was maintained at 450 8C during the deposition. The typical deposition time was 3 h and the ®lm thickness was approximately 2 mm. XPS measurements were carried out on a PHI5702XPS/AES spectrometer using Mg Ka radiation of 1253.6 eV as an excitation source. XRD spectroscopy was obtained on a Rigaku D/Max-IIIC X-ray
339
diffractometer using Cu Ka1 radiation. FT-IR spectra were measured using a WQF-400 spectrophotometer and SEM measurement was conducted on a JEOL JSM-5600LV apparatus. 3. Results and discussion In order to determine the chemical states of the elements in the deposited BCN ®lms, XPS measurements were ®rst carried out for the samples deposited on Si under different conditions. Fig. 1 shows the XPS spectra of B 1s, N 1s and C 1s for a typical BCN ®lm. It can be seen that the XPS peaks of B 1s, N 1s and C 1s binding energies are centered at 190.2, 398.5 and 284.2 eV, and the corresponding full width at half maximum (FWHM) of the peaks are 3.2, 2.6 and 2.8 eV, respectively. The broadening of the peaks suggests that the atoms were incorporated into the ®lms with more than one type of the bonding schemes. Therefore, the XPS spectra were resolved to obtain the possible chemical bonding existing in the sample. As shown in Fig. 1(a), the deconvolution of B 1s spectrum gives two peaks centered at 189.5 and 190.8 eV, respectively. It was reported that the B 1s spectra have peaks at 188.4 and 189.4 eV for B4C and BC3.4, respectively, and 191.0 eV for h-BN ®lms [12]. Thus, the resolved two peaks can be attributed to B±C and B±N bonding, implying that the B atoms in the ®lms bond with both C and N atoms. In Fig. 1(b), the results of the deconvolution indicate that there are two types of N chemical states in the ®lm, and the peaks of their binding energies are centered at 398.4 and 399.4 eV, respectively. The former can be assigned to the N±B bonding and the latter to the N±C bonding [12]. The deconvolution of the C 1s spectrum gives two peaks centered at 284.2 and 285.6 eV as shown in Fig. 1(c). Considering that the binding energy of C 1s involved in ethylamine is 285.2 eV, this allows us to assign the peak at 285.6 eV to C±N bonding. Since C 1s peaks in BC2N and BC3.4 center at 284.4 and 284.3 eV, respectively [12], and the peak in graphite almost appears at the same position (284.4 eV), the deconvolution peak at 284.2 eV can be regarded as a combined contribution of both C±B and C±C bonding. XPS results described above suggest that the BCN ®lms are atomic-level hybrids composed of B, C and N
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D. He et al. / Applied Surface Science 191 (2002) 338±343
Fig. 2. The typical XRD pattern for BC2N ®lms.
Fig. 1. XPS spectra of (a) B 1s, (b) N 1s and (c) C 1s for a representative BC2N ®lm.
atoms. The atomic fractions in the ®lms were estimated from the integral areas of the XPS peaks calibrated by the corresponding sensitivity factors. For the sample shown in Fig. 1, it was found that the atomic percentages of C, B and N are 54, 26 and 20 at.%, respectively, which is close to the stoichiometry of BC2N. For the samples deposited at different CH4 ¯ow rates, XPS measurements showed that more
Fig. 3. FT-IR spectra for BC2N ®lms on Si substrate (a) and peeled off the substrate (b).
D. He et al. / Applied Surface Science 191 (2002) 338±343
C atoms are incorporated into the ®lms with increasing the CH4 ¯ow rate. Fig. 2 shows a typical XRD spectrum of the samples deposited on fused silica plate. Two peaks at 2y 25:1 and 43.88 can be seen, which coincide with the reported diffraction data of BC2N [7,13], indicating that the ®lms have a graphite-like structure. The calculated in-plane and c-axis lattice constants are 0.24 and 0.69 nm, respectively. A slight decrease of the in-plane lattice constant and an increase of the caxis constant compared with those of graphite, as well
341
as an asymmetrical broadening of the diffraction peak at 2y 43:8 may imply the existence of the internal stress in the ®lms. To investigate the internal stress in the BCN ®lms, the FT-IR absorption spectra were measured for the ®lms on Si substrate and peeled off the substrate. The FT-IR absorption measurement has been widely used to analyze the residual stress in diamond and c-BN thin ®lms since it was found that the absorption bands would shift due to the stress in the ®lms [14]. Fig. 3(a) shows the FT-IR absorption spectrum for an
Fig. 4. SEM images of BC2N ®lms deposited on Si wafer (a) and fused silica plate (b).
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D. He et al. / Applied Surface Science 191 (2002) 338±343
as-deposited BC2N ®lm on Si substrate. Four absorption bands and a shoulder can be observed at 617, 776, 1110, 1272 and 1370 cm 1, respectively. The band at 617 cm 1 comes from the infrared vibration absorption of the Si wafer, and the other two at 776 and 1370 cm 1 can be attributed to the in-plane B±N stretching and the out-of-plane B±N±B bending modes, respectively. Because the stretching mode of C±N is in the range from 1250 to 1360 cm 1, the band at 1272 cm 1 should be attributed to the absorption of the C±N vibration. In addition, considering that the C± B stretching vibration is in the range from 1090 to 1200 cm 1, the weak absorption at 1110 cm 1 should be contributed to C±B stretching mode. The present FTIR data support the results obtained by XPS, con®rming that the ®lms are atomic-level hybrids composed of B, C and N atoms. The FT-IR spectrum was greatly changed once the ®lm was peeled off the Si substrate. The peel-off of the ®lms implies that the stress within the ®lms has been relieved to some extent. As shown in Fig. 3(b), the absorption band coming from the out-of-plane B±N±B bending mode is intensi®ed signi®cantly and shifts to 749 cm 1. Moreover, a peak at 1089 cm 1 and a band centered at 1270 cm 1 can be observed. From the shape and strength of the peak at 1089 cm 1, we regard it as a superposition of the bands at 1123 and 1033 cm 1 which belong to N±C and B±C stretching modes, respectively. Thus, it can be concluded that all bands, especially for those due to the out-of-plane B±N±B bending mode and vibration absorption related to C atoms, shift toward lower wavenumbers when the ®lm is peeled off the substrate and the stress within the ®lms relieves to some extent. SEM measurements con®rmed that there is considerable compressive stress in the ®lms. Fig. 4 shows the surface morphologies of the BC2N ®lms on Si and fused silica substrates. The compressive stress makes the ®lms buckle from the substrates. For the ®lms on the Si substrates a branching type stress relief pattern was observed, while a telephone-cord morphology is formed on the ®lm deposited on fused silica substrates. The period of oscillation of the telephone-cord delamination was found to be very regular. Many others have also observed such a telephonecord morphology delamination for compressively stressed thin ®lms [15]. Based on energy methods,
Oritiz and coworkers [16,17] have developed a buckling-driven delamination theory to interpret the phenomenon. A straight delamination relieves stress in only one direction, however if the tip undulates back and forth as in the telephone-cord morphology, a biaxial stress state can be effectively relieved. Their analysis predicted that the transition from straight-sided to telephone-cord morphologies occurs at a critical mismatch strain which is related to the used substrate. 4. Conclusions Graphitic BC2N thin ®lms were prepared by radio frequency reactive sputtering. XPS and FT-IR measurements suggested that the ®lms are atomic-level hybrids composed of B, C and N atoms. High compressive stresses were found within the ®lms. When the ®lms are peeled off the substrate, stress relieves and the IR absorption shifts to lower wavenumbers. The stress relief depends on the used substrates. On silicon substrates a branching type stress relief pattern was observed, while a telephone-cord morphology was noted on fused silica plates. The buckling patterns can be qualitatively explained using the bucklingdriven delamination theory. Acknowledgements This work was supported in part by the National Natural Science Foundation of China, and in part by the University Key Teacher Foundation of the Education Ministry of China. References [1] A.Y. Liu, R.M. Wentzcovitch, M.L. Cohen, Phys. Rev. B 39 (1989) 1760. [2] M. Kawaguchi, Adv. Mater. 9 (1997) 615. [3] E.H.A. Dekempeneer, J. Meneve, S. Kuypers, J. Smeets, Thin Solid Films 281±282 (1996) 331. [4] S. Nakano, M. Akaishi, T. Sasaki, S. Yamaoka, Mater. Sci. Eng. A 209 (1996) 26. [5] M. Morita, T. Hanada, H. Tsutsumi, Y. Matsuda, M. Kawaguchi, J. Electrochem. Soc. 139 (1992) 1227. [6] R. Riedel, J. Bill, G. Passing, Adv. Mater. 3 (1991) 551. [7] M. Kawaguchi, T. Kawashima, J. Chem. Soc., Chem. Commun. (1993) 1133.
D. He et al. / Applied Surface Science 191 (2002) 338±343 [8] M.O. Watanabe, S. Itoh, K. Mizushima, T. Sasaki, J. Appl. Phys. 78 (1995) 2880. [9] A. Perrone, A.P. Caricato, A. Luches, M. Dinescu, C. Ghica, V. Sandu, A. Andrei, Appl. Surf. Sci. 133 (1998) 239. [10] S. Fayeulle, M. Nastasi, J. Appl. Phys. 81 (1997) 6703. [11] J. Yue, W. Cheng, X. Zhang, D. He, G. Chen, Thin Solid Films 375 (2000) 247. [12] M.O. Watanabe, T. Sasaki, S. Itoh, K. Mizushima, Thin Solid Films 281±282 (1996) 334.
343
[13] Y.G. Andreev, T. Lundstrom, J. Alloys Comp. 210 (1994) 311. [14] S. Ulrich, J. Scherer, J. Schwan, I. Barzen, K. Jung, M. Scheib, H. Ehrhardt, Appl. Phys. Lett. 68 (1996) 909. [15] B.J. Daniels, W.D. Nix, B.M. Clemens, Mater. Res. Soc. Symp. Proc. 328 (1995) 315. [16] M. Oritiz, G. Gioia, J. Mech. Phys. Solids 42 (1994) 531. [17] G. Gioia, M. Oritiz, Acta Mater. 46 (1998) 169.
Relief of the internal stress in ternary boron carbonitride thin ®lms Deyan Hea,*, Wenjuan Chenga, Juan Qina, Jinshun Yuea, Erqing Xiea, Guanghua Chenb b
a Department of Physics, Lanzhou University, Lanzhou 730000, China College of Materials Science and Engineering, Beijing Polytechnic University, Beijing 100022, China
Received 30 October 2001; accepted 27 March 2002
Abstract Ternary boron carbonitride (BCN) thin ®lms were deposited by radio frequency reactive sputtering. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) absorption measurements suggested that the ®lms are atomic-level hybrids composed of B, C and N atoms. The structure of the samples is graphitic BC2N. High compressive stresses were observed in the ®lms deposited on silicon and fused silica substrates. When the ®lms were peeled off the substrates, the absorption bands in the FT-IR spectra shift to lower wavenumbers due to the stress relief. The compressive stresses caused buckling of the ®lms, and the buckling patterns were related to the used substrates. For the samples deposited on Si wafer, a branching type buckling pattern was observed, while a telephone-cord morphology was noted for the samples deposited on fused silica plate. The telephone-cord morphology can be explained based on the buckling-driven delamination theory proposed by Oritiz and Gioia [J. Mech. Phys. Solids 42 (1994) 531]. # 2002 Elsevier Science B.V. All rights reserved. PACS: 68.35.Gy; 68.55.Jk Keywords: Ternary boron carbonitride thin ®lms; Compressive stress; Buckling pattern; Buckling-driven delamination theory
1. Introduction Although graphite and hexagonal boron nitride (hBN) are both hexagonal close-packed in structure and have similar atomic size and crystal constant, they differ greatly in properties: graphite is a good conductor, while h-BN is an insulator. The structural similarity between the two materials inspires materials scientists to develop a ternary BCN compound. The structure of the compound is expected to be graphite-like, and *
Corresponding author. Tel.: 86-931-891-2546; fax: 86-931-891-3554. E-mail address: [email protected] (D. He).
its properties, which may be between those of graphite and h-BN, like a semiconductor with adjustable band gap by controlling the atomic composition and arrangement [1,2]. On the other hand, since ternary BCN compounds can be prepared in a wide composition range, the materials are also expected to be used as protective coatings in tribology and optics [3] and precursors for the preparation of heterodiamond under high-pressure high-temperature conditions [4]. Moreover, it was shown that the intercalation property of BCN compounds is between the graphite and h-BN, and the materials could be applied for rechargeable lithium batteries [5].
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 2 5 4 - 4
D. He et al. / Applied Surface Science 191 (2002) 338±343
Much effort has been made to prepare BCN compounds. It was reported that the material could be synthesized by various techniques [6±9] and the resultant samples were stoichiometrically varied: BCN, BC2N, BC3N, and B2C5N, etc. Some published works also focused on the structural characterization of the materials. With regard to the applications of BCN compounds as protective coatings in tribology and optics, their mechanical behavior and stability are of primary importance. High compressive stress has been found in BCN ®lms [10]. Since the compressive stress may affect the adhesion properties of the materials on substrates and thereby their applications as protective coatings, it is necessary to have an understanding of the stress relief within the materials. In this paper, BCN thin ®lms were prepared by radio frequency reactive sputtering method. The microstructure and the residual stress relief within the samples were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) absorption, and scanning electronic microscope (SEM). 2. Experimental The BCN ®lms used in this work were deposited by reactive sputtering technique [11]. The target was a sintered h-BN disk with a diameter of 10 cm. The sputtering gas was a mixture of methane and argon. The fused silica plate and high resistance Si wafer were used as substrates. Before the deposition, the substrates were rinsed ultrasonically in acetone, ethanol and de-ionized water and then dried by N2 blowing. For Si wafer, a dip in a solution of 5% HF was done just before mounting it onto the acceptor in order to remove the surface thin oxide layer. The ¯ow rate ratio of CH4/Ar was 15% with the total ¯ow being 20 sccm. The working pressure was kept at 1 10 2 Torr. The density of the radio frequency power was 3.84 W/cm2 and the substrate temperature was maintained at 450 8C during the deposition. The typical deposition time was 3 h and the ®lm thickness was approximately 2 mm. XPS measurements were carried out on a PHI5702XPS/AES spectrometer using Mg Ka radiation of 1253.6 eV as an excitation source. XRD spectroscopy was obtained on a Rigaku D/Max-IIIC X-ray
339
diffractometer using Cu Ka1 radiation. FT-IR spectra were measured using a WQF-400 spectrophotometer and SEM measurement was conducted on a JEOL JSM-5600LV apparatus. 3. Results and discussion In order to determine the chemical states of the elements in the deposited BCN ®lms, XPS measurements were ®rst carried out for the samples deposited on Si under different conditions. Fig. 1 shows the XPS spectra of B 1s, N 1s and C 1s for a typical BCN ®lm. It can be seen that the XPS peaks of B 1s, N 1s and C 1s binding energies are centered at 190.2, 398.5 and 284.2 eV, and the corresponding full width at half maximum (FWHM) of the peaks are 3.2, 2.6 and 2.8 eV, respectively. The broadening of the peaks suggests that the atoms were incorporated into the ®lms with more than one type of the bonding schemes. Therefore, the XPS spectra were resolved to obtain the possible chemical bonding existing in the sample. As shown in Fig. 1(a), the deconvolution of B 1s spectrum gives two peaks centered at 189.5 and 190.8 eV, respectively. It was reported that the B 1s spectra have peaks at 188.4 and 189.4 eV for B4C and BC3.4, respectively, and 191.0 eV for h-BN ®lms [12]. Thus, the resolved two peaks can be attributed to B±C and B±N bonding, implying that the B atoms in the ®lms bond with both C and N atoms. In Fig. 1(b), the results of the deconvolution indicate that there are two types of N chemical states in the ®lm, and the peaks of their binding energies are centered at 398.4 and 399.4 eV, respectively. The former can be assigned to the N±B bonding and the latter to the N±C bonding [12]. The deconvolution of the C 1s spectrum gives two peaks centered at 284.2 and 285.6 eV as shown in Fig. 1(c). Considering that the binding energy of C 1s involved in ethylamine is 285.2 eV, this allows us to assign the peak at 285.6 eV to C±N bonding. Since C 1s peaks in BC2N and BC3.4 center at 284.4 and 284.3 eV, respectively [12], and the peak in graphite almost appears at the same position (284.4 eV), the deconvolution peak at 284.2 eV can be regarded as a combined contribution of both C±B and C±C bonding. XPS results described above suggest that the BCN ®lms are atomic-level hybrids composed of B, C and N
340
D. He et al. / Applied Surface Science 191 (2002) 338±343
Fig. 2. The typical XRD pattern for BC2N ®lms.
Fig. 1. XPS spectra of (a) B 1s, (b) N 1s and (c) C 1s for a representative BC2N ®lm.
atoms. The atomic fractions in the ®lms were estimated from the integral areas of the XPS peaks calibrated by the corresponding sensitivity factors. For the sample shown in Fig. 1, it was found that the atomic percentages of C, B and N are 54, 26 and 20 at.%, respectively, which is close to the stoichiometry of BC2N. For the samples deposited at different CH4 ¯ow rates, XPS measurements showed that more
Fig. 3. FT-IR spectra for BC2N ®lms on Si substrate (a) and peeled off the substrate (b).
D. He et al. / Applied Surface Science 191 (2002) 338±343
C atoms are incorporated into the ®lms with increasing the CH4 ¯ow rate. Fig. 2 shows a typical XRD spectrum of the samples deposited on fused silica plate. Two peaks at 2y 25:1 and 43.88 can be seen, which coincide with the reported diffraction data of BC2N [7,13], indicating that the ®lms have a graphite-like structure. The calculated in-plane and c-axis lattice constants are 0.24 and 0.69 nm, respectively. A slight decrease of the in-plane lattice constant and an increase of the caxis constant compared with those of graphite, as well
341
as an asymmetrical broadening of the diffraction peak at 2y 43:8 may imply the existence of the internal stress in the ®lms. To investigate the internal stress in the BCN ®lms, the FT-IR absorption spectra were measured for the ®lms on Si substrate and peeled off the substrate. The FT-IR absorption measurement has been widely used to analyze the residual stress in diamond and c-BN thin ®lms since it was found that the absorption bands would shift due to the stress in the ®lms [14]. Fig. 3(a) shows the FT-IR absorption spectrum for an
Fig. 4. SEM images of BC2N ®lms deposited on Si wafer (a) and fused silica plate (b).
342
D. He et al. / Applied Surface Science 191 (2002) 338±343
as-deposited BC2N ®lm on Si substrate. Four absorption bands and a shoulder can be observed at 617, 776, 1110, 1272 and 1370 cm 1, respectively. The band at 617 cm 1 comes from the infrared vibration absorption of the Si wafer, and the other two at 776 and 1370 cm 1 can be attributed to the in-plane B±N stretching and the out-of-plane B±N±B bending modes, respectively. Because the stretching mode of C±N is in the range from 1250 to 1360 cm 1, the band at 1272 cm 1 should be attributed to the absorption of the C±N vibration. In addition, considering that the C± B stretching vibration is in the range from 1090 to 1200 cm 1, the weak absorption at 1110 cm 1 should be contributed to C±B stretching mode. The present FTIR data support the results obtained by XPS, con®rming that the ®lms are atomic-level hybrids composed of B, C and N atoms. The FT-IR spectrum was greatly changed once the ®lm was peeled off the Si substrate. The peel-off of the ®lms implies that the stress within the ®lms has been relieved to some extent. As shown in Fig. 3(b), the absorption band coming from the out-of-plane B±N±B bending mode is intensi®ed signi®cantly and shifts to 749 cm 1. Moreover, a peak at 1089 cm 1 and a band centered at 1270 cm 1 can be observed. From the shape and strength of the peak at 1089 cm 1, we regard it as a superposition of the bands at 1123 and 1033 cm 1 which belong to N±C and B±C stretching modes, respectively. Thus, it can be concluded that all bands, especially for those due to the out-of-plane B±N±B bending mode and vibration absorption related to C atoms, shift toward lower wavenumbers when the ®lm is peeled off the substrate and the stress within the ®lms relieves to some extent. SEM measurements con®rmed that there is considerable compressive stress in the ®lms. Fig. 4 shows the surface morphologies of the BC2N ®lms on Si and fused silica substrates. The compressive stress makes the ®lms buckle from the substrates. For the ®lms on the Si substrates a branching type stress relief pattern was observed, while a telephone-cord morphology is formed on the ®lm deposited on fused silica substrates. The period of oscillation of the telephone-cord delamination was found to be very regular. Many others have also observed such a telephonecord morphology delamination for compressively stressed thin ®lms [15]. Based on energy methods,
Oritiz and coworkers [16,17] have developed a buckling-driven delamination theory to interpret the phenomenon. A straight delamination relieves stress in only one direction, however if the tip undulates back and forth as in the telephone-cord morphology, a biaxial stress state can be effectively relieved. Their analysis predicted that the transition from straight-sided to telephone-cord morphologies occurs at a critical mismatch strain which is related to the used substrate. 4. Conclusions Graphitic BC2N thin ®lms were prepared by radio frequency reactive sputtering. XPS and FT-IR measurements suggested that the ®lms are atomic-level hybrids composed of B, C and N atoms. High compressive stresses were found within the ®lms. When the ®lms are peeled off the substrate, stress relieves and the IR absorption shifts to lower wavenumbers. The stress relief depends on the used substrates. On silicon substrates a branching type stress relief pattern was observed, while a telephone-cord morphology was noted on fused silica plates. The buckling patterns can be qualitatively explained using the bucklingdriven delamination theory. Acknowledgements This work was supported in part by the National Natural Science Foundation of China, and in part by the University Key Teacher Foundation of the Education Ministry of China. References [1] A.Y. Liu, R.M. Wentzcovitch, M.L. Cohen, Phys. Rev. B 39 (1989) 1760. [2] M. Kawaguchi, Adv. Mater. 9 (1997) 615. [3] E.H.A. Dekempeneer, J. Meneve, S. Kuypers, J. Smeets, Thin Solid Films 281±282 (1996) 331. [4] S. Nakano, M. Akaishi, T. Sasaki, S. Yamaoka, Mater. Sci. Eng. A 209 (1996) 26. [5] M. Morita, T. Hanada, H. Tsutsumi, Y. Matsuda, M. Kawaguchi, J. Electrochem. Soc. 139 (1992) 1227. [6] R. Riedel, J. Bill, G. Passing, Adv. Mater. 3 (1991) 551. [7] M. Kawaguchi, T. Kawashima, J. Chem. Soc., Chem. Commun. (1993) 1133.
D. He et al. / Applied Surface Science 191 (2002) 338±343 [8] M.O. Watanabe, S. Itoh, K. Mizushima, T. Sasaki, J. Appl. Phys. 78 (1995) 2880. [9] A. Perrone, A.P. Caricato, A. Luches, M. Dinescu, C. Ghica, V. Sandu, A. Andrei, Appl. Surf. Sci. 133 (1998) 239. [10] S. Fayeulle, M. Nastasi, J. Appl. Phys. 81 (1997) 6703. [11] J. Yue, W. Cheng, X. Zhang, D. He, G. Chen, Thin Solid Films 375 (2000) 247. [12] M.O. Watanabe, T. Sasaki, S. Itoh, K. Mizushima, Thin Solid Films 281±282 (1996) 334.
343
[13] Y.G. Andreev, T. Lundstrom, J. Alloys Comp. 210 (1994) 311. [14] S. Ulrich, J. Scherer, J. Schwan, I. Barzen, K. Jung, M. Scheib, H. Ehrhardt, Appl. Phys. Lett. 68 (1996) 909. [15] B.J. Daniels, W.D. Nix, B.M. Clemens, Mater. Res. Soc. Symp. Proc. 328 (1995) 315. [16] M. Oritiz, G. Gioia, J. Mech. Phys. Solids 42 (1994) 531. [17] G. Gioia, M. Oritiz, Acta Mater. 46 (1998) 169.